1. Field of the Invention
This invention relates to a halftone printing system for thermal printing and thermal transfer printing. More particularly, it relates to a halftone printing system in which the change of printing density attributed to the heat accumulation hysteresis of a thermal head can be corrected, to thereby faithfully reproduce the density corresponding to each gradation level.
2. Description of the Prior Art
Since a conventional thermal printing device or thermal transfer printing device has a comparatively simple construction, it is extensively applied to various kinds of printing means such as printers, copying machines, and facsimile sets. In such various printing means, the method of thermal transfer printing with a sublimation type inked sheet, for example, is sometimes used for the printing of halftones. The thermal transfer printing method performs required printing in such a way that dye ink is sublimated in correspondence with the quantity of heat applied by heating resistors which constitute the thermal head, whereupon the sublimated dye ink is transferred onto a paper. Here, the heating resistors have their heating quantity controlled by the number and duration of electric pulses applied thereto.
The thermal transfer printing method is easy to control, and can achieve comparatively good halftone printing. In the halftone printing, however, the main factor which determines the printing densities of a plurality of gradation levels is the temperature of the heating resistors constituting the thermal head. Therefore, the printing density of each gradation level is greatly affected by changes in ambient temperature and temperature fluctuation ascribable to the heat accumulation of the heating resistors. Thus, it is difficult to perform faithful halftone printing, and a large number of correcting methods have hitherto been proposed.
A halftone printing system in the prior art is disclosed in, for example, Japanese Patent Application Laid-open No. 60-9271. FIGS. 1 and 2 are diagrams for explaining the operation of this prior-art system.
First, FIG. 1 is a waveform diagram of electric pulses (hereinbelow, called a "strobe signal") which serve to heat heating resistors constituting a thermal head in the prior-art system. Here, symbol t.sub.w denotes the pulse width of the strobe signal, symbol t.sub.p the repetition period of the strobe signal, and letter n the number of pulses of the strobe signal used. Further, the number of pulses of the strobe signal to be used is previously selected and set in correspondence with the density of each gradation level, and a case where the number is 3 is illustrated in FIG. 1.
Next, FIG. 2 is a diagram of the relationship of the prior-art system between the pulse width of the strobe signal applied to the heating resistor and the temperature of the heating resistor. Here, the pulse width t.sub.w of the strobe signal is the ordinate, while a changeable temperature such as ambient temperature is the abscissa. By controlling the pulse width of the strobe signal in correspondence with the temperature change, an energization or heating time for the heating resistors constituting the thermal head is shortened in inverse proportion to the temperature rise, and the temperature at the identical gradation level can be always held constant.
In the operation of the prior-art system, even when the number of corresponding pulses is fixed in order to attain a certain desired gradation level, the printing density sometimes fluctuates under the influence of the temperature in the heating resistors constituting the thermal head. That is, notwithstanding that the number of pulses corresponding to the gradation level remains constant, the printing density becomes different. Therefore, while the temperature change of the ambient temperature etc. is being monitored every line of printing by means of an affixed appropriate temperature sensor such as a thermistor (not shown), the pulse width t.sub.w of the strobe signal is controlled in the aspect as shown in FIG. 2. In this way, corrections are made so as to attain equal printing densities at a certain desired gradation level by the same number of pulses.